Hitherto, in the fields of chemical analysis, biochemical analysis, and the like, a surface plasmon resonance (SPR) sensor including an optical fiber has been used. In the SPR sensor including an optical fiber, a metal thin-film is formed on an outer circumferential surface of a tip end portion of the optical fiber, and an analysis sample is fixed to the optical fiber into which light is guided. Of the light to be guided, light having a particular wavelength generates surface plasmon resonance in the metal thin-film, and light intensity thereof is attenuated. In such an SPR sensor, the wavelength of the light generating surface plasmon resonance generally varies depending on a refractive index of an analysis sample to be fixed to the optical fiber. Therefore, if a wavelength at which light intensity is attenuated after the generation of surface plasmon resonance is measured, the wavelength of the light generating surface plasmon resonance can be identified. Further, if a change in the wavelength at which light intensity is attenuated is detected, it can be confirmed that the wavelength of the light generating surface plasmon resonance has changed, and hence a change in the refractive index of the analysis sample can be confirmed. As a result, such an SPR sensor can be used for various chemical analyses and biochemical analyses such as measurement of a sample concentration and detection of an immunoreaction.
In the SPR sensor including an optical fiber, the tip end portion of the optical fiber has a fine cylindrical shape, and hence there is a problem in that it is difficult to form the metal thin-film and fix an analysis sample to the optical fiber. In order to solve the problem, for example, there has been proposed an SPR sensor cell including a core through which light is transmitted and a clad covering the core, in which a through-hole extending to a surface of the core is formed at a predetermined position of the clad, and a metal thin-film is formed on the surface of the core at a position corresponding to the through-hole (for example, Patent Literature 1). In such an SPR sensor cell, it is easy to form the metal thin-film for generating surface plasmon resonance on the surface of the core and fix the analysis sample onto the surface. Further, it has been proposed that the detection accuracy of the SPR sensor cell can be enhanced by continuously changing the refractive index of a region on an under-cladding layer side in the core layer in the SPR censor cell (Patent Literature 2).
However, in recent years, in chemical analysis and biochemical analysis, there is an increasing demand for detection of a minute change and/or a trace amount of component, and thus further enhancement of detection sensitivity of the SPR sensor cell is being demanded.